(Invited) Investigating Microstructural Effects of Composite NCM811 Cathodes on ALL-Solid-State Li-Ion Batteries Performance Under Low Operation Stacking Pressure

Abstract

All-solid-state lithium-ion batteries (ASSLIBs) have been considered suitable alternatives to commercial lithium-ion batteries (LIBs) in the aspect of safety issues that come from the use of inflammable solid electrolytes to replace organic flammable electrolytes. Nevertheless, there apparently remains much room for a better understanding of their properties and behaviors in order to upgrade their performance to reach the practical application level. Ni-rich layered oxides, LiNi1-x-yCoxMnyO2 (NCM), are promising cathodes for high-energy ASSLIBs because of their high capacities and redox potentials, and low material cost when compared with conventional LiCoO2. However, certain challenges associated with their use in ASSLIBs must be addressed for their effective use and industrialization. In particular, the structural integrity of the NCM electrodes during the long-term charge/discharge process suffers from the formation of an intergranular and/or intragranular cracking behavior. These microcracks are initiated at the grain boundaries from the anisotropic chemical strain of the randomly oriented polycrystal (PC) primary cathode particles. Solid evidence behind the electrochem-mechanical breakdown of an NCM cathode with liquid electrolytes has been well discussed and reported in the literature. However, the breakdown in ASSLIBs may not obey as same as in the liquid-electrolyte LIBs and display exclusive mechanics-oriented material damage in the presence of rigid solid-state electrolytes (SSEs). The primary objective of this research is to gain a deeper understanding of various cracking modes in ASSLIBs, which we believe is instrumental to improving the lifespan and cell performance as a whole. ForASSLIBs, stacking pressures greater than 100 MPa under battery operation are currently commonly used. The use of high stacking pressure increases manufacturing and usage difficulties and overall cost. Lowering the battery-operation stacking pressure, for example to the level of the liquid-electrolyte batteries, will certainly facilitate the wider applications of ASSLIBs. In this context, composite cathodes consisting of a LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode and brittle Li3InCl6 (LIC) SSE have been assessed for ASSLIB applications under low operation stacking pressure (coin-cell configuration; ~2.0 MPa). The study reveals the effects of material and structural characteristics of the composite cathode, including the NCM particle size and size distribution, active-material spatial distribution, particle crystallinity (polycrystal or single-crystal), and electrochemical operation parameters (voltage range, current rates) on the cycle stability of the ASSLIBs as a result of the electrochem-mechanical behaviors of the oxide cathode. Meanwhile, a novel solvent-based synthesis process of LIC for achieving high-density cathodes is introduced.